Vacuum tube voltmeter circuit



Dec. 12, 1944. J R BANKER 2,364,687

VACUUM TUBE VOLTMETER CIRCUIT Filed April 24, 1945 A 17 21 22 25 b /I 2 7 25 M fi%/@wm/E-m HITORNE 7 Patented Dec. 12, 1944 UNITED STATES PATENT OFFICE 2,864,687 VACUUM 'runn VOLTMETER cmcurr John Richard Banker, Passaic, N. .L, assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a corporation of Delaware Application April 24, 1943, Serial No. 484,498

10 Claims. (Cl. 171-95) ured. In using the device for measuring alternating voltages a vacuum tube degenerative amplifier is followed by a diode rectifier and a sensitive microammeter.

The invention may be understood from the description in connection with the accompanying drawing, in which reference character I indicates the input terminal or the voltage that is to be measured. It is connected to the grid 2 of a tube 2 which may be a triode, for example. It is provided with grid leak 4. The plate 5 is con nected to a source 8 of direct current voltage.

The cathode 8 of tube 3 is connected by resist ance 1 to the negative end of source 8 of voltage which may be a duplicate of source 8. The positive end or this source 8 and the negative end of the source 8 are grounded. The resistance I caruses an IR drop across resistance 1. The contacting point In is moved to the zero potential on resistance 1 with respect to ground, when the switch I2 is closed. This zero position point is the point of contact of contactor III on resistance 1 where no current fiow's through microammeter M.

When D. C. positive potential is applied to terminal I, an increase of current fiows through tube 3 causing more current to flow through resistance I and battery 8 thus raising the potential of cathode 6 and thus raising the potential at point l0 which causes current to flow through resistance [0 and microammeter M when the switch I2 is closedat terminal ll.

When'the switches I2 and 21 are turned to the right hand positions to connect with the contacts 25 and 28, respectively, the microammeter M is ready to read the rectified current which is generated when A. C. voltage is applied to the grid 2.

A. C. voltage applied to grid 2 causes an A. C. voltage of the same frequency to appear at the cathode 6. This A. C. voltage at the cathode 6 is coupled by condenser l4 and lead I5 to the plate iii of rectifier I1 and is rectified thus removing the positive half of the signal. Any direct current component that may reach the cathode 6 is eliminated by the condenser Hi. The rectified current, which is the negative half of the signal,

is connected by a sliding contactor Ill on resistp e through resistance 5 d is indicated by ance 1 through resistance In to a terminal H which may be connected by switch l2 to one side of a microammeter M of which the other side is grounded. The part thus far described is for measuring direct current voltages.

For measuring alternating current voltages the cathode 6 is coupled by condenser l4 and lead I 5 to the plate It of diode H. The cathode l8 of this diode is connected by lead is to the plate 20 of a second diode 2|, the cathode of which is connected through resistance 23 by lead 24 to terminal 25. A resistance 26 is connected between terminal 25 and lead IS. The switch l2 which may make contact with either terminal I l or 25 is connected to microammeter M and is ganged with the switch 21 which may be contacted with terminal 28 to make ground connection at the end or lead It or plate 20, and also at the cathode l8. Or this switch 21 may make contact with the contact 29. This switch is left in its neutral position when the device is not in use.

When the switches I2 and 21 are turned to the left hand positions with zero potential on the meter M. The emission current of diode I! which would otherwise pass through microammeter M and cause an incorrect reading is balanced by the emission current through diode 2| and resistance 23 when the switch l2 makes contact with terminal 25 so that only the current that passes through the resistance 26 also passes through the meter M. This meter is calibrated to read in volts so that very small alternating voltages applied at the input terminal I result in producing easily read deflections of the meter M.

What is claimed is:

1. A voltmeter circuit, comprising a vacuum tube having a plate, a grid and a cathode, and also comprising a fixed resistance in the cathode circuit of said tube, a meter and a resistance in series between a variable point on said first named resistance and ground, and terminals for connecting sources of direct current potential from ground to said plate and said fixed resistance, respectively.

2. The circuit of claim 1, in which means for applying a voltage to be measured to the grid of said tube is used.

grid 2, current through tube 2 and resistance I 5 3. The device of claim 1, in which the positive end of one of said sources of potential is grounded.

4. The device of claim 1, in which said vacuum tube is a triode.

5. The device of claim 1, in which said fixed resistance and source of potential are so proportioned and connected that a point on said first named resistance is at zero potential when no potential is applied to the grid of said tube.

6. A voltmeter circuit, comprising a vacuum tube having a plate, a grid and a cathode, and also comprising a fixed resistance in the cathode circuit of said tube. a condenser, resistance and meter in series between said cathode and ground and in parallel with said first named resistance, terminals for connecting sources 0! direct current potential from ground to said plate and said fixed resistance, respectively, and a diode connected between the junction of said condenser and said last named resistance and ground.

7. The device of claim 6, in which a diode is also provided to balance the emission current of the first named diode.

8. The device of claim 6, in which a diode is also provided to balance the emission current of the first named diode, and means are provided to ground the cathode of the first named diode and the plate of the other one.

9. A voltmeter circuit, comprising a vacuum tube having a plate, a grid and a cathode, and also comprising a fixed resistance in the cathode circuit of said tube, a meter, a switch and a resistance in series between said cathode and ground in one position of said switch, said meter and switch being in parallel with said first named resistance in another position of said switch, terminals for connecting sources of direct current potential from ground to said plate of said first named resistance, respectively, and means comprising diodes to adapt the circuit for reading alternating current voltages in said last named position of said switch.

10. The device of claim 6, in which means is provided to remove any direct current component when alternating voltages are being measured.

JOHN RICHARD BANKER. 

